Polydipsia and hyponatremia in patients with mental illness

Psychiatric patients, particularly those with schizophrenia, often have abnormalities in water balance. As an example, one study of 239 hospitalized patients found that 6.6 percent had a history compatible with compulsive water drinking and that one-half of these had intermittent symptoms of hyponatremia due to transient water retention. However, a higher percentage of patients may have milder defects in water homeostasis.

ABNORMALITIES IN WATER BALANCE

Evaluation of psychotic patients has revealed that a variety of defects in water handling can occur, affecting thirst, the release of antidiuretic hormone (ADH), and the renal response to ADH. Depending upon which abnormality is present, the patient may present with polydipsia and polyuria and/or hyponatremia.

Primary polydipsia — Many chronically psychotic patients have a moderate to marked increase in water intake. This may be manifested clinically by exaggerated weight gain during the day (2.2 percent versus 0.6 percent in normal controls in one study) , that is associated with a transient reduction in the plasma sodium concentration. (Accurate 24-hour urine collections are often difficult to obtain in psychotic patients; as a result, the mean daytime weight gain is used as an index of increased intake.)

It is presumed that a central defect in thirst regulation plays an important role in the pathogenesis of polydipsia . In some cases, for example, the osmotic threshold for thirst is reduced below the threshold for the release of antidiuretic hormone (ADH). These patients will continue to drink until the plasma tonicity is less than the threshold level. (The plasma tonicity refers to that portion of the total plasma osmolality that generates an osmotic pressure; in most cases, tonicity is determined by the concentration of the nonurea solutes.) This may be difficult to achieve, however, since ADH secretion will be suppressed by the fall in plasma tonicity, resulting in rapid excretion of the excess water and continued stimulation of thirst.
The osmotic regulation of thirst is different from that in normal subjects in whom the thirst threshold is a roughly equal to or a few mosmol/kg higher than the threshold for ADH.